In my own understanding and based from what i've experienced, an act can be called addiction when you always do such things whether you're happy or not. You just can't resist on doing so.
Hobby is a doing that you that you can stop whenever you wanted to.
Sometimes these two words are misunderstood. But in my point of view, the meaning of this words are dependent to those who will use it to describe an act.
What can you say about the differences of these words?